1. Field of the Invention
The present invention relates to a DC/DC (DC is an abbreviation for "Direct Current") converter which converts a given DC input voltage to a desired DC output voltage for supplying an electric load, by means of a switching control of an switching element, e.g., by executing a pulse width modulation of such a switching element.
More specifically, the present invention relates to a DC/DC converter, which enables higher efficiency for a voltage conversion to be attained, even in the case where an electric load connected to an output terminal of the DC/DC converter is light and requires only a small electric current, with a relatively simple circuit.
Further, the present invention relates to a DC/DC converter of step-down voltage type, in which a field-effect (FET) transistor having a relatively low resistor value in an ON-state can be used as the switching element, so that a relatively high efficiency for a voltage conversion and a relatively small whole size can be ensured.
2. Description of the Related Art
Generally, a DC/DC converter utilizing a switching operation, e.g., by means of a PWM (Pulse-Width Modulation) control of a switching element such as a transistor has a relative small size, and therefore can be constituted by an IC (integrated circuit). Such an IC has usually a relatively wide frequency range in regard to operational oscillation frequencies for the PWM control.
Typically, the IC can oscillate even with an oscillation frequency as high as 500 kHz. By using such a high oscillation frequency, a choke coil, a capacitor, and the like, utilized as main circuit components of a smoothing circuit in the DC/DC converter, can be smaller in their size. Therefore, the whole size of the DC/DC converter, which is constituted by an IC oscillating with a higher oscillation frequency, is likely to be remarkably small.
Here, to enable circuit configurations of DC/DC converters of the prior art to be easily understood, some examples of conventional DC/DC converters will be described with reference to the related drawings of FIGS. 1 to 4.
FIG. 1 is a circuit diagram showing a first example of a DC/DC converter of the prior art.
In FIG. 1, C10 and C20 denote capacitors; S10 denotes a switching element, e.g., a transistor; D10 denotes a diode; and L10 denotes an inductance coil, e.g., a choke coil. Further, 110 denotes a control unit which output a PWM signal for controlling an ON-OFF operation of the switching element S10 (i.e., for switching control), on the basis of detection result of an output DC voltage from the DC/DC converter.
In such a construction, when a given DC input voltage is supplied to an input terminal of the DC/DC converter, the DC input voltage can be converted to a desired DC output voltage for sending from an input terminal of the DC/DC converter to an electric load, by a control function of the control unit 110.
FIG. 2 is a circuit diagram showing a circuit configuration of a control unit in FIG. 1.
In FIG. 2, a control unit 110 includes an amplifier(AMP) 112 which is usually referred to as an error amplifier, and a comparator(COMP) 113 which is connected to an output of the amplifier 112.
More specifically, the amplifier 112 is operative to compare the DC output voltage with a predetermined reference voltage Vref and to amplify the voltage difference between the DC output voltage and the predetermined reference voltage Vref. The comparator 113 is operative to compare the voltage difference output from the amplifier 112 with a voltage of a predetermined saw tooth wave.
Further, the operation of the DC/DC converter of the prior art shown in FIGS. 1 and 2 will be described.
In the control unit 110, as shown in FIG. 2, the DC output voltage from the DC/DC converter is compared with the reference voltage Vref by means of the amplifier 112. Further, a signal, which corresponds to the voltage difference between the DC output voltage and the predetermined reference voltage Vref, is supplied to the comparator 113. Subsequently, the comparator 113 outputs a PWM signal, in accordance with a comparison result that is obtained by comparing the voltage difference output from the amplifier 112 with a voltage of a predetermined saw tooth wave. The switching element S10 executes an ON-OFF operation, in accordance with the PWM signal which is output from the comparator 113.
In this case, the capacitor C10 shown in FIG. 1 is charged in advance by means of a DC input voltage which is supplied to an input of the DC/DC converter. When the switching element S10 becomes ON-state by the PWM signal which is output from the control unit 110, a DC input voltage of the DC/DC converter is supplied to an inductance coil L10, through the switching element S10. Therefore, an electric energy is accumulated in this inductance coil L10. Also, another capacitor C20 is charged by means of the DC input voltage which passes through the inductance coil L10.
On the other hand, when the switching element S10 becomes OFF-state by the above-mentioned PWM signal, an electric energy, which has been accumulated during ON-state duration of the switching element S10, is released, through the capacitor C20 and the diode D10. Consequently, the capacitor C20 is further charged by means of the thus released electric energy.
In the case where a voltage between both terminals of the capacitor C20 become higher than the predetermined reference voltage Vref in the control unit 110, a pulse width of each pulse of the PWM signals decreases. Therefore, ON-state duration of the switching element S10, corresponding to the time period during which the switching element becomes ON-state, is shortened. On the other hand, in the case where a voltage between both terminals of the capacitor C20 become lower than the reference voltage Vref in the control unit 110, a pulse width of each pulse of the PWM signals increases. Therefore, ON-state duration of the switching element S10 is lengthened. In both cases, an DC output voltage of the DC/DC converter is controlled by the control unit 110 so that a voltage value of the DC output voltage becomes equal to a voltage value of the reference voltage Vref.
FIG. 3 is a circuit diagram showing a second example of a DC/DC converter of the prior art. Hereinafter, any component that is the same as that mentioned before will be referred to using the same reference number.
In FIG. 3, a DC/DC converter of step-down voltage type(for example, a voltage value of a DC input voltage is 6 V to 18 V, while a voltage value of a DC output voltage is 5 V) is illustrated, in which a P-channel type field-effect transistor Q10 is used as a circuit element corresponding to the switching element S10 shown in FIG. 1. Further, in FIG. 3, a Schottky barrier diode SBD is used as a circuit element corresponding to the diode D10 shown in FIG. 1.
Further, in the figure, a switch circuit 120, which is mainly constituted by a bipolar transistor Q20, is provided at the output side of the DC/DC converter. The switch circuit 120 includes resistors R20, R30 for adequately adjusting operating characteristics of the bipolar transistor Q20. Further, the switch circuit 120 includes another bipolar transistor Q30 for supplying an adequate electric current to the bipolar transistor Q20; and other resistors R40, R50 for determining an operating point of the bipolar transistor Q30. The switch circuit 120 is necessary to make a voltage level of the DC output voltage stable even in a DC/DC converter of step-down voltage type.
FIG. 4 is a circuit diagram showing a third example of a DC/DC converter of the prior art.
In this case, an N-channel type field-effect transistor Q11 is provided, in place of a P-channel type field-effect transistor Q10 in a DC/DC converter of step-down voltage type shown in FIG. 3. Also, another N-channel type field-effect transistor Q21 is provided, in place of a bipolar transistor Q20 in the switch circuit 120. Each of these N-channel type field-effect transistors usually has a relatively low resistor value in an ON-state, and therefore a higher efficiency for a DC voltage conversion can be attained.
However, in this example, it should be noted that it is necessary for a DC/DC converter circuit 130 of step-up voltage type to be additionally provided, in order to obtain a driving voltage source for driving the N-channel type field-effect transistor Q21 in the switch unit 120. The DC/DC converter circuit 130 has a control switching unit 132 including a switching element and a control unit, an inductance coil L30, and a capacitor C30, similar to a circuit configuration of FIG. 1.
As described above, in the DC/DC converter shown in each of FIGS. 1 and 2, a switching operation, i.e., an ON-OFF operation of a switching element is executed by utilizing a PWM signal which is obtained by comparing a voltage output from an error amplifier with a voltage of a predetermined saw tooth wave, as a control signal. Here, in the case where an electric load connected to an output terminal of the DC/DC converter is changed, only a duty factor of each pulse in a PWM signal is changed, and a switching frequency of the PWM signal remains constant.
Further, in the above-mentioned DC/DC converter, as an oscillation frequency of the PWM signal becomes higher, a power loss which is mainly generated at the timing of switching of the switching element remarkably increases, and therefore an efficiency for a DC voltage conversion is deteriorated.
Especially, even in the case where an electric load connected to an output terminal of the DC/DC converter is light and requires only a small current, the PWM signal oscillates with an approximately constant oscillation frequency.
Here, it is assumed that a power loss in one period of switching of the switching element in the PWM signal is constant, independent of characteristics of an electric load. Further, it is assumed that an electric power of a desired DC output voltage is supplied from a battery to an apparatus having this electric load. In the case where an electric power required for an electric load ranges from several milliwatts (mW) to several thousand milliwatts, when an electric power required for an electric load is in a middle range and a higher range, a relatively high efficiency for a DC voltage conversion is obtained.
However, when such an electric power is in a lower range (for example, from several milliwatts to several tens of milliwatts), an efficiency for a DC voltage conversion is remarkably deteriorated. Especially, when an electric power as low as several milliwatts continues to be consumed for a relatively long time, the battery is likely to be rapidly exhausted.
Therefore, the DC/DC converter shown in each of FIGS. 1 and 2 has a disadvantage that the time for which a battery, etc., can be used is likely to be shortened, in the case where an electric power supplied to an electric load is relatively low.
Generally, in a DC/DC converter of step-down voltage type, a control voltage of a switching element is lower than a DC input voltage of the DC/DC converter. Therefore, as already shown in FIG. 3, a P-channel type field-effect transistor or a bipolar PNP transistor having a relatively high resistor value in an ON-state has been used as a switching element.
Further, to attain a higher efficiency for a DC voltage conversion, it has been attempted to use an N-channel type field-effect transistor having a relatively low resistor value in an ON-state, as already shown in FIG. 4. Usually, to operate such an N-channel type field-effect transistor, a positive gate voltage equal to or more than 10 V has to be applied to a gate terminal, and also higher driving voltage has to be applied to a drain terminal. Therefore, it becomes necessary for a DC/DC converter circuit of step-up voltage type to be provided, in order to obtain a higher driving voltage source (the driving voltage is Vcc) for driving the N-channel type field-effect transistor.
However, in this case, an extra DC/DC converter circuit of step-up voltage type is necessary to attain a higher efficiency for a DC voltage conversion. Therefore, the DC/DC converter shown in FIG. 3 has a disadvantage that the whole circuit size of the DC/DC converter becomes larger. Further, owing to the extra DC/DC converter circuit of step-up voltage type, a cost for fabricating such a DC/DC converter as in FIG. 3 is likely to increase.
On the other hand, in the case where the P-channel type field-effect transistor or a bipolar PNP transistor is used as the switching element, as shown in FIG. 3, the switching element has a relatively high resistor value in an ON-state. Therefore, the DC/DC converter shown in FIG. 3 has a disadvantage that it becomes difficult to attain a sufficiently high efficiency for a DC voltage conversion.